1. Field of the Invention
The present invention relates to a light source apparatus capable of changing an oscillation wavelength thereof and also relates to an imaging apparatus equipped with the light source apparatus.
2. Description of the Related Art
In the field of communication networks as well as in the field of inspection apparatuses, a light source apparatus capable of changing an oscillation wavelength is well known. Recent developments in these fields have raised the need to increase a wavelength sweep rate and widen a sweep band of the light source apparatus.
As an example of an inspection apparatus, there is an optical coherence tomography (OCT) apparatus configured to capture a tomographic image of an inspection target based on optical coherence. OCT is a non-invasive imaging technique, which can be widely practiced in the medical field because of its noninvasiveness. Recently, a new technique for OCT, known as swept source OCT (SS-OCT) has been actively investigated.
A swept source optical coherence tomography (SS-OCT) apparatus uses a wavelength sweep light source to obtain depth information based on spectral interference. SS-OCT is useful in that the loss of light quality is small because no spectroscope is required and an image having a signal-to-noise (S/N) ratio higher than in time domain OCT can be acquired.
In the SS-OCT apparatus, it is desired to increase the wavelength sweep rate because the time required to acquire an image can be reduced. Specifically, if the wavelength sweep rate is high, a vital observation can be made in a relatively short time, thereby reducing an influence of inevitable movement of the object under observation.
Further, it is desired to broaden the band because the spatial resolution of a tomographic image can be increased if the wavelength sweep band is wide.
In general, the following formula (1) can be used to represent the resolution of a tomographic image in the depth direction, in which Δλ represents a wavelength sweep width and λ0 represents an oscillation wavelength.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                                  2            ⁢            ln            ⁢                                                  ⁢            2                    π                ×                              λ            0            2                                Δ            ⁢                                                  ⁢            λ                                              Formula        ⁢                                  ⁢                  (          1          )                    
Accordingly, increasing the wavelength sweep width Δλ is useful to increase the resolution in the depth direction.
A light source usable for an SS-OCT apparatus is discussed in non-patent literature (NPL) document entitled “Wide and fast wavelength-tunable mode-locked fiber laser based on dispersion tuning”, by S. Yamashita, et al. Opt. Exp. Vol. 14, pp. 9299-9305 (2006) (hereinafter, referred to as “NPL 1”). As described in NPL 1, the conventional light source usable for the SS-OCT apparatus performs dispersion tuning for realizing a wavelength variation (sweeping) based on wavelength dispersion (hereinafter, simply referred to as “dispersion”) of the refractive index of a material in a resonator. The resonator uses a semiconductor optical amplifier (SOA) that has been studied at a band to be chiefly used in the communication field.
The above-described dispersion tuning includes controlling the oscillation wavelength in an active mode locking state because a free spectral range (hereinafter referred to as “FSR”) of the resonator has wavelength dependence. More specifically, the dispersion tuning includes performing a wavelength sweeping operation by changing the frequency of a modulation signal that causes the active mode locking. Therefore, a high-speed wavelength sweeping operation can be realized by quickly changing the frequency of the modulation signal.
In this case, the free spectral range indicates the frequency interval of a resonator mode relative to the light circulating in the resonator. The free spectral range (FSR) can be defined by the following formula (2), in which c represents the speed of light in the vacuum, n represents a refractive index of the resonator, and L represents a resonator length.
                    [                  Formula          ⁢                                          ⁢          2                ]                                                            FSR        =                  c          nL                                    Formula        ⁢                                  ⁢                  (          2          )                    
Further, as discussed in NPL 1, the wavelength sweep range Δλ according to the dispersion tuning can be defined by the following formula.
                    [                  Formula          ⁢                                          ⁢          3                ]                                                                      Δ          ⁢                                          ⁢          λ                =                  n          cDN                                    Formula        ⁢                                  ⁢                  (          3          )                    where N is a positive integer indicative of the order of harmonic mode locking, and D is the dispersion parameter in the resonator.
According to the dispersion tuning based on the active mode locking discussed in NPL 1, the wavelength sweep rate can be increased by quickly changing the frequency of the modulation signal. However, the wavelength sweep range is dependent on the semiconductor optical amplifier (SOA) that constitutes the resonator. Therefore, the sweep range is limited by the wavelength dispersion in the resonator cannot be sufficiently broadened.
On the other hand, the U.S. patent application Ser. No. 7,443,903 (hereinafter, referred to as patent literature “PTL 1”), there is a conventional laser apparatus configured to synchronously irradiate a plurality of positions with laser light. According to PTL 1, light emitted from one master laser can be guided to a plurality of amplifiers in such a way as to synchronously control a plurality of output heads.
The laser apparatus discussed in PTL 1 can output synchronized light beams from the plurality of output heads via a plurality of amplifiers coupled with the same master laser. However, the laser apparatus discussed in PTL 1 does not perform any operation for sweeping the wavelength of the output light.